1. Field of the Invention
The present invention relates to a capacitive gas sensor and a method of fabricating the same. More specifically, the present invention relates to a capacitive gas sensor that uses a complex oxide nano structure and simplifies the structure two-dimensionally, and a method of fabricating the same.
2. Discussion of Related Art
A recent increase in concerns about environmental 20 contamination and health remarkably has led to an increase in necessity for detection of harmful gases. Gas sensors originally developed to meet demands for detection of toxic gases and explosive gases are now being developed to meet demands for enhancement of the quality of human life in the fields of health management, environmental monitoring, industrial health and safety, home appliances and home automation, food and agriculture, manufacturing processes, and national defense and terrorism. Therefore, gas sensors may become a means by which a future society free of disasters can be implemented, and more accurate measurement and control of environmentally harmful gases are required.
In order for gas sensors to be practical, high sensitivity, high selectivity, long-term stability, and high response characteristics are required, as are low power consumption and high integration density. In order to satisfy these requirements, gas sensors using various sensor structures and materials and fabrication methods are being developed.
Gas sensors using ceramics are mainly classified into semiconductor gas sensors, solid electrolyte gas sensors, and contact combustion gas sensors, which are distinguished from each other by their types, structures, and materials.
In particular, when oxide semiconductor ceramic such as zinc oxide (ZnO), tin oxide (SnO2), tungsten oxide (WO3), titanium oxide (TiO2), or indium oxide (In2O3) contacts an environmental gas such as H2, CO, O2, CO2, NOx, a toxic gas, a volatile organic gas, ammonia, an environmental gas, or humidity, electrical resistivity is changed due to adsorption of gases and oxidation/reduction reactions occurring on a surface of the metal oxide. Therefore, studies on resistance type gas sensors using these characteristics are being carried out, and some of the resistance type gas sensors have already been commercialized.
In recent years, many studies on development of gas sensors using physical characteristics of nano structures such as nano thin films, nano particles, nano wires, nano fibers, nano tubes, nano pores, and nano belts different from the characteristics of bulk materials are under way. Small sizes and extremely large surface-to-volume ratios of the nano structures enable manufacture of sensors of rapid reaction time and ultra-high sensitivity. The new materials enable development of gas sensors having excellent characteristics such as quick response speed, high sensitivity, high selectivity, and low power consumption.
However, although a resistance type gas sensor using an oxide semiconductor of a nano structure can achieve very high sensitivity, it does not guarantee high selectivity, long-term stability, or high reproducibility due to instable contact resistance and instability to an external environment.
A conventional oxide semiconductor gas sensor includes a substrate, an oxide sensing material, a metal electrode transducer for detecting an electrical signal of a sensor, and a micro thin-film heater. The micro thin-film heater is located on the top or bottom surface of a thin film and has a structure independent from the metal electrode transducer. This makes the fabrication process of the gas sensor complex.
FIG. 1 is a schematic view of a conventional metal oxide semiconductor gas sensor having a membrane structure. The gas sensor of FIG. 1 is fabricated by the following process. First, after a first insulating layer 105 is formed on a support substrate 106, micro thin-film heater wires 103 are disposed on the first insulating layer 105 and then a second insulating layer 104 is formed on the structure. Then, the support substrate 106 is etched to expose the bottom surface of the first insulating layer 105. Here, the etching of the support substrate 106 is carried out to avoid loss of heat and integrate heat. Electrodes 102 and an oxide layer 101 are deposited on the structure.
It can be understood that, when a gas sensor having a micro heater is to be fabricated, a plurality of lithography processes are necessary to realize the micro heater of the gas sensor. Electrical heating wires located in an insulating layer need to be electrically coupled through additional etching and metal deposition processes in order to apply a voltage therethrough. The etching of a support substrate also requires a very complex process. Although a membrane type micro heater is excellent in collection of heat, a silicon wafer needs to be etched by its general thickness, i.e. 600 microns, making the process complex.
Therefore, development of new sensor materials and sensors compensating for the disadvantages of conventional gas sensors realized by oxide semiconductor materials and having excellent characteristics such as high sensitivity, high selectivity, quick response speed, and long-term stability are urgently required.
Metal oxide semiconductor ceramic, a thin film, and a nano structure such as zinc oxide (ZnO), tin oxide (SnO2), tungsten oxide (WO3), titanium oxide (TiO2), or indium oxide (In2O3) are known as favorable materials for development of resistance type environmental gas sensors using electrical resistivity characteristics varied by adsorption of gases and oxidation/reduction reactions occurring on a surface of the metal oxide when they are in contact with an environmental gas.
In addition, many studies on ceramic mixtures of an oxide of BaTiO3 and metal oxides such as CaO, MgO, NiO, CuO, SnO2, MgO, La2O3, Nd2O3, Y2O3, CeO2, PbO, ZrO2, Fe2O3, Bi2O3, V2O5, Nb2O5, and Al2O3 and ceramic mixtures of different metal oxides such as WO3—(ZnO, CuO, NiO, SnO2, MgO, Fe2O3), NiO—(V2O5, SrTiO3, ZnO, In2O3, BaSnO3), ZnO—(SnO2, In2O3), and CoO—In2O3 are under way. The electrostatic capacity or impedance of such a complex oxide material is changed by adsorption of gases and oxidation/reduction reactions occurring on a surface of a metal oxide since it is in contact with an environmental gas, so that the complex oxide material is a favorable material for development of a capacitive gas sensor.
Such a capacitive gas sensor compensates for the disadvantages of conventional resistance type oxide semiconductor gas sensors and is driven by an AC voltage. The capacitive gas sensor enables low power consumption, high sensitivity, high selectivity, quick gas reaction rate, simplification of its fabrication process due to its simple structure, and miniaturization, and particularly enables long-term stability against an external environment and high integration density. In addition, in the capacitive gas sensor, amplification of electrical capacity can be easily realized by an oscillator circuit and price can be lowered due to a simple signal processing circuit.
Accordingly, the present inventors, while studying a capacitive gas sensor, discovered that the capacitive gas sensor can be easily fabricated due to simplification of its structure when a metal electrode and a micro thin-film heater are integrally formed on the same plane, thereby compensating for the disadvantages of conventional resistance type oxide semiconductor gas sensors.